1. Field of the Invention
The present invention relates to an apparatus for suppressing a peak amplitude of a transmission signal in a wireless communication device.
2. Description of the Related Art
In a wireless communication device, a power amplifier amplifies a generated signal and transmits the resultant signal. Since an amplifier consumes a lot of power, it is desired to use the amplifier with high power use efficiency. However, while the power use efficiency of an amplifier is the more powerfully enhanced when the amplifier is used where the output power is the higher, it has the feature that the output is saturated. When the amplifier is saturated, distortion of its output signal occurs, the spread of a spectrum occurs outside the frequency band of a transmission signal, thereby causing an interference wave to other frequencies. It is important to efficiently use the amplifier in a limited input voltage range without distortion.
Thus, there has been a method of transmitting a signal with the lowest possible peak average per ratio (PAPR) to the average power of a transmission signal because, if the limited input voltage range of an amplifier is kept, and a signal of high average power is amplified, an amplifier can be used with high operation efficiency without causing distortion. The conventional peak suppressing method proposed for a transmission signal and realized by combining the hard clipping (HC) in nonlinear processing and filtering (low pass filter (LPF)).
FIG. 1 is an explanatory view of a conventional peak suppressing method.
In the hard clipping process, the hard clipping is performed on a suppression target value for a transmission signal having the amplitude exceeding a suppression target value. That is, the amplitude exceeding the suppression target value is forcibly cut off. In the filtering process, the component outside the signal frequency band of the transmission signal generated in the hard clipping process is cut off. The amplitude of the peak component is low relative to the signal before suppression.
The patent document 1 relates to the conventional method of improving the efficiency of the power amplifier. In the patent document 1, the envelope of a transmission signal is suppressed such that the set level cannot be exceeded, the signal band is limited by a filter, the signal is modulated, and then amplified by the power amplifier                [Patent Document 1] Japanese Patent Application Publication No. H10-178414        
As shown in FIG. 1, when peak suppression is performed by combining the conventional hard clipping and filtering, there occurs the problem that a peak is rebuilt after passing the LPF (the amplitude exceeds the suppression target value again).
Therefore, to suppress the peak of a signal to desired amplitude, the following two methods have been conventionally used.
(1) Method of suppressing the peak by strongly performing the HC processing with the peak rebuilding after passing the LPF taken into account
(2) Method of suppressing the rebuilt peak by repeatedly performing the process of (HC+LPF)
However, in (1) above, there is the problem that the modulation precision to the amount of peak suppression and the degradation of the bit error rate increase. In (2) above, there is the problem that the circuit is large, thereby requiring larger power consumption and increased process delay.